1. Field of the Invention
The present invention relates to an improved sol-gel method for the preparation of multicomponent monolithic oxide glasses having high concentration of non-SiO.sub.2 components.
2. Discussion of the Background
The conventional methods of preparing multicomponent silicate glasses involve mixing solid precursor powders, followed by melting the mixture of powders. However, the use of solid-phase materials limits the homogenity achievable in the mixture of powders, which in turn, limits the homogeneity of glasses produced therefrom. Furthermore, the conventional solid precursor powders used in the preparation of multicomponent monolithic oxide glasses are generally metal oxides, which are very difficult to purify, and which have dangerously high melting temperatures. Furthermore, the molten metal oxide precursor powders often separate into different phases, resulting in a further reduction in the homogeneity of the glasses produced by conventional melting methods.
Recently, significant interest has developed in producing modified silica gel which contains at least one metal other than silicon. A method using a mixed solution of a silicon alkoxide and at least one additional metal alkoxide has been suggested for the production of such modified silica glass (Kimiya et al, "Proceeding of the Tenth International Congress on Glass," Kyoto, Japan (July 1974), Part II, pages 13-44 to 13-48).
Many applications in optics and in other fields require glasses with a high refractive index. In order to achieve a high refractive index, it is desirable to incorporate elements such as lead or the rare earths into the glass. However, these elements are difficult to use in the mixed alkoxide method because their corresponding alkoxides are either unstable or only sparingly soluble in the solvents conventionally employed.
Other glass components which raise the refractive index, such as titanium, zirconium or niobium, are convenient to use in the mixed alkoxide method. One example is the mixed alkoxide method of U.S. Pat. No. 4,278,632 (incorporated herein by reference), used to produce silica-titania glasses. However, in order to achieve high concentrations of these elements while simultaneously avoiding devitrification, it is necessary to include relatively large amounts of alkaline elements, such as potassium or sodium. Unfortunately, when alkaline elements are used in the mixed alkoxide method, the resulting precursor gel is often very fragile and prone to fracture.